The various compounds that are derived from the polymerization of .alpha.-olefins show huge differences in their chemical and physical properties. These differences reflect differences in molecular structure, some of which are inherent in the use of a particular monomer or monomer combination, and some of which result from a pattern, or lack thereof, in how the monomers are combined. It is inherent that polymers of .alpha.-olefins having 3 or more carbon atoms will have pendant hydocarbyl groups attached to the polymer backbone chain. However, the stereochemical arrangement of these hydrocarbyl groups is a consequence of the interaction of monomer, catalyst and coordinated polymer during polymerization. Any pendant hydrocarbyl group may be said to lie on one side of a plane defined by the carbon atoms of the polymer backbone in an idealized elongated configuration.
As previously alluded to, the physical properties exhibited by a particular olefin polymer of a particular molecular weight are determined in major part by the arrangement of these hydrocarbyl groups along the polymer backbone. Strong polymers tend to be stereochemically regular, meaning the adjacent hydrocarbyl groups reside on the same side of the polymer backbone or switch at fairly regular intervals. Either arrangement facilitates crystalization thus lending ridgidity and strength to the the solidified polymer.
Other critical determinants of the properties which a polymer will exhibit are the type and relative concentration of monomers and comonomers, the weight average molecular weight (M.sub.w) of the polymer molecules comprising the resin bulk, the molecular weight distribution (MWD) and the composition distribution of the resin. For end use applications which require high strength and low creep, the M.sub.w of such a resin must generally be in excess of 100,000.
Five types of stereoregularity, or tacticity, have been characterized: atactic, normal isotactic, isotactic stereoblock, syndiotactic, and hemiisotactic. Although all of these stereoregular configurations have been primarily demonstrated in the case of polypropylene, in theory each is equally possible for polymers comprised of any olefin, cyclic olefin or internal olefin, having 3 or more carbon atoms.
Atactic polyolefins are those wherein the hydrocarbyl groups pendent to the polymer molecule backbone assume no regular order in space with reference to the backbone. This random structure is represented by a polymer backbone of alternating methylene and methine carbons, with randomly oriented branches substituting the methine carbons. The methine carbons randomly have R and S configurations, creating adjacent pairs either of like configuration (a "meso" or "m" dyad) or of unlike configuration (a "racemic" or "r" dyad). The atactic form of a polymer contains approximately equal fractions of meso and racemic dyads. Since atactic polyolefins exhibit no regular order or repeating unit configurations in the polymer chain, they are amorphous materials. Atactic polyolefins exhibit little if any crystallinity, hence they are generally unsuitable for high strength applications regardless of the weight average molecular weight of the resin.
Isotactic polyolefins are those wherein the pendent hydrocarbyl groups are ordered in space to the same side or plane of the polymer backbone chain. Using isotactic polypropylene as an example, the isotactic structure is typically described as having the pendent methyl groups attached to the ternary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the carbon backbone chain of the polymer, e.g., the methyl groups are all above or below the plane as shown below. ##STR1## The degree of isotactic regularity may be measured by NMR techniques. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane.
In the normal isotactic structure of a polyolefin, all of the monomer units have the same stereochemical configuration, with the exception of random errors which appear along the polymer. Such random errors almost always appear as isolated inversions of configuration which are corrected in the very next .alpha.-olefin monomer insertion to restore the original R or S configuration of the propagating polymer chain.
The formation of stereoblock isotactic polymer differs from the formation of the normal isotactic structure in the way that the propagation site reacts to a stereochemical error in the chain. As mentioned above, the normal isotactic chain will return to the original configuration following an error because the stereochemical regulator, the catalytic active metal species and its surrounding ligands, continues to dictate the same stereochemical preference during monomer insertion. In stereoblock propagation, the catalytically active metal site itself changes from one which dictates a monomer insertion of R configuration to one which dictates an S configuration for monomer insertion. The isotactic stereoblock form is shown below. ##STR2##
Long before anyone had discovered a catalyst system which produced the isotactic stereoblock form of a polyolefin, the possible existence of a polymer of such micro-structure had been recognized and mechanisms for its formation had been proposed based on conventional Ziegler-Natta mechanisms in Langer, A. W., Lect. Bienn. Polym. Symp. 7th (1974); Ann. N.Y. Acad. Sci. 295, 110-126 (1977). The first example of this form of polypropylene and a catalyst which produces it in a pure form were reported in U.S. Pat. No. 4,522,982.
The lengths of individual blocks of the same configuration in the stereoblock structure vary widely due to changing reaction conditions. Since only the erroneous parts of the chains affect the crystallinity of the resin product, in general, normal isotactic polymers and isotactic stereoblock polymers of long block length (greater than 50 isotactic placements) have similar properties.
Highly isotactic polyolefins exhibit a high degree of crystallinity. Accordingly, isotactic polyolefins are, depending upon their weight average molecular weight exceeding about 100,000, well suited to high strength end use applications.
Syndiotactic polyolefins are those wherein the hydrocarbyl groups pendent to the polymer molecular backbone alternate sequentially in order from one side or plane to the opposite side or plane relative to the polymer backbone, as shown below. ##STR3## In NMR nomenclature, this segment, or pentad, is described as . . . rrrr . . . in which each r represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Highly syndiotactic polymers are generally highly crystalline and will frequently have high melting points similar to their isotactic polymorphs. Like isotactic polyolefins, syndiotactic polyolefins are capable of exhibiting a high degree of crystallinity, hence are suitable for high strength applications provided their M.sub.w exceeds about 100,000.
For any of the above described materials the final resin properties and its suitability for particular applications depend on the type of tacticity, the melting point, the average molecular weight, the molecular weight distribution, the type and level of monomer and comonomer, the sequence distribution, and the presence or absence of head or end group functionality. Accordingly, the catalyst system by which such a stereoregular polyolefin is to be produced should, desirably, be versatile in terms of M.sub.w, MWD, tacticity type and level, and comonomer choice. Further, the catalyst system should be capable of producing these polymers with or without head and/or end group functionality, such as olefinic unsaturation. Still further, such catalyst system must be capable, as a commercially practical constraint, of producing such resins at an acceptable production rate. Most preferably, the catalyst system should be one which, at its productivity rate, provides a resin product which does not require a subsequent treatment to remove catalyst residue to a level which is acceptable for the resin in the end use application desired. Finally, an important feature of a commercial catalyst system is its adaptability to a variety of processes and conditions.
Conventional titanium based Ziegler-Natta catalysts for the preparation of isotactic polymers are well known in the art. These commercial catalysts are well suited for the production of highly crystalline, high molecular weight materials. The systems are, however, limited in terms of molecular weight, molecular weight distribution, and tacticity control. The fact that the conventional catalysts contain several types of active sites further limits their ability to control the composition distribution in copolymerization.
More recently a new method of producing isotactic polymers from an alumoxane cocatalyzed, or activated, metallocene which in its natural state has chirality centered at the transition metal of the metallocene, was reported in Ewen, J. A., J. Amer. Chem. Soc., v. 106, p. 6355 (1984) and Kaminsky, W., et al., Angew. Chem. Int. Ed. Eng.; 24, 507-8 (1985).
Catalysts that produce isotactic polyolefins are also disclosed in U.S. Pat. No. 4,794,096. This patent discloses a chiral, stereorigid metallocene catalyst which is activated by an alumoxane cocatalyst which is reported to polymerize olefins to isotactic polyolefin forms. Alumoxane cocatalyzed metallocene structures which have been reported to polymerize stereoregularly are the ethylene bridged bis-indenyl and bis-tetrahydroindenyl titanium and zirconium (IV) catalyst. Such catalyst systems were synthesized and studied in Wild et al., J. Organomet. Chem. 232, 233-47 (1982), and were later reported in Ewen and Kaminsky et al., mentioned above, to polymerize .alpha.-olefins stereoregularly. Further reported in West German Off DE 3443087Al (1986), but without giving experimental verification, is that the bridge length of such stereorigid metallocenes can vary from a C.sub.1 to C.sub.4 hydrocarbon and the metallocene rings can be simple or bi-cyclic but must be asymmetric. In contrast to the metallocene catalysts disclosed in U.S. U.S. Pat. No. 4,794,096 and West German Patent DE 3443087Al, certain species of this invention are capable of producing a highly isotactic polymer using an achiral catalyst.
The use of transition metal based catalysts having amido groups attached to the transition metal have received some attention in polymer research. A process for producing syndiotactic polystyrene using a tris-amido zirconium catalyst is disclosed in U.S. Pat. No. 4,774,301. As taught therein, the zirconium compound may be combined with an alumoxane to produce a polymerization catalyst. A syndiotactic polymer results when vinyl aromatic monomers, which have been known to yield syndiotactic polymers generally, are polymerized by a tris-amido zirconium-alumoxane catalyst.
In European Patent 349,886, titanium having bonded thereto a saturated alkyl-substituted amido group, is reported to yield an active catalyst in the presence of alumoxane. This catalyst system is capable of producing polyethylene copolymers having a high degree of structural randomness and narrow molecular weight distribution. Also reported in EP 349,886 are references to prior amide containing Group IV-B metal catalysts for the homopolymerization of ethylene, which, when applied to copolymerization, suffer from the various disadvantages of low molecular weight product, broad molecular weight distribution and low catalytic activity.
In view of the high strength and other physical properties that make stereoregular polymers desirable in applications for which other moldable plastics are ill suited and in view of the few methods currently available for producing stereoregular polymers, there is a need for a catalyst as disclosed hereinafter for producing high molecular weight, highly isotactic polymers. It is further desirable that such a catalyst have high activity so as to allow production of a polymer which is ready to be molded and/or machined for its ultimate use without treatment for removal of contaminants (catalyst residue). It is also desirable to obtain a catalyst that is useful for production of ethylene base polymers.